Systematic variations in the wavelength dependence of interstellar linear polarization

New observations of the wavelength dependence of interstellar linear polarization, p(lambda), which were performed to investigate the influence of the environment on the effective size distribution of the aligned polarizing particles, are presented. Optical and IR measurements were obtained simultaneously in eight photometric passbands between U and K, giving a coherent data set for a total of 105 reddened stars. The contention that variations of K and lambda-max are correlated is confirmed, and the empirical linear relationship between K and lambda-max found by Wilking et al. (1980, 1982, 1983) is revised to K + 0.01 +/-0.05 + (1.66 +/-0.09)lambda-max. There is some cosmic scatter of the data about this line. The same linear dependence of K on l-max seen overall provides a consistent representation of the systematic polarization in changes within individual regions with rather differing environments. It is argued that the grain size distribution in dense regions is modified by coagulation which removes the smaller particles without major modification of the larger ones.